The present invention relates to a logging tool used to search for underground mineral or fossil fuel deposits and, more particularly, to a method and apparatus for determining the electrical conductivity of formations proximate to a borehole.
Electrical conductivity (or its inverse, resistivity) is an important property of a rock formation in geological surveys and prospecting for oil and gas because many minerals, and more particularly hydrocarbons, are less conductive than common sedimentary rocks. Thus a measure of the conductivity is often a guide to the presence and amount of oil or gas.
Induction methods using coils to generate and sense time-varying electromagnetic fields are widely used in borehole geophysical surveys, or "logs", to determine the local rock properties including conductivity, dielectric permittivity, and magnetic permeability. Typically, arrays of such coils mounted coaxially with the borehole axis, and operating in the frequency range from 5 KHz to 200 KHz (typically about 20 KHz) are used to sense the conductivity, while frequencies up to 200 MHz may be used to determine dielectric permittivity. The magnetic properties, while valuable, are not commonly measured by these methods due to the difficulty of separating their effects from those of conductivity.
Recent development of directional and horizontal drilling techniques allow the borehole to be "steered" while drilling in order to follow the boundaries of an oil-rich formation for a considerable distance instead of merely intercepting it, as was traditionally the case. This method is leading to revolutionary changes in recovery rates of oil and gas, combined with reduced drilling costs. Conductivity measuring tools and other gravity and magnetometer sensors are typically combined in a "measurement while drilling" (MWD) arrangement to provide a stream of data concerning the location and quality of hydrocarbon deposits while drilling. The data may also be used during completion of the well, when placement of casing perforations is being decided, by indicating regions where hydrocarbon saturation or producibility may be too low for completion. In addition, logs may be used to monitor a producing well that is not cased and to indicate the approach of water or gas boundaries (e.g. coning) during workover jobs. Generally, information about the location of the geological and fluid boundaries of a reservoir are very valuable in determining the total volume of hydrocarbon reserves.
For measurement-while-drilling applications, in particular, it is desirable to provide a logging tool that senses primarily to one side of the borehole and to a selected distance from the borehole, so that the proximity of an upper or lower boundary of a reservoir formation may be sensed before the drill-bit has penetrated through it, and in time for corrective action to be taken to modify the path of the bit through the formation. It follows that a method of sensing contrasting rock properties at the greatest distance in a selected direction would provide a distinct advantage. Of all the sensing means in common use, such as acoustic, nuclear and electrical, the wireline induction method has the greatest depth of investigation (up to five feet). However, the problem of adapting the induction technique to measurement-while-drilling has been found to be difficult, due to the influence of the mass of conductive metal in a drill-collar, and there are no known true MWD induction tools (directional or not) in commercial operation at this time. All existing MWD resistivity tools, other than those using electrodes, are based on a relatively high-frequency method commonly described as "wave propagation". These are induction tools that operate in a relatively high frequency range (typically 0.4 to 2 MHz) where the phenomenon of skin-effect dominates the propagation of the electromagnetic energy between coils, due to the conductivity and magnetic permeability of the nearby rock formations. Unfortunately, this limits the depth of investigation to significantly less than what a true induction tool can achieve and does not provide sufficient depth to modify the path of the drill bit before it has penetrated the formation.
While there is a continuing need for an induction tool for use in MWD, there are no commercially acceptable tools or services of this type available at this time. Various attempts have been made to place a standard wireline induction tool inside a non-conductive collar, usually made of a fiberglass-epoxy composite material. Such materials have successfully been used in drill-pipe and are commercially available from Brunswick Composites of Lincoln, Nebr. Unfortunately, the requirements for drill collars are much more severe than for drill pipe in terms of mechanical stresses (axial, torsional, and bending combined), and resistance to the abrasive effect of drill-cuttings and contact with the borehole wall. These environmental hazards lead to a short life for non-metal collars, particularly at junctions with metal collars that have higher rigidity.
U.S. Pat. No. 5,442,294 (Rorden) describes a method for placing coils in slots at various positions around the periphery of a drill collar at spaced-apart distances along the axis of the collar, to cancel the transmitter primary magnetic field, rather than the more usual induction tool arrangement of mutually-balanced coil arrays. Analysis shows that the Rorden method will suffer from the problem of significant errors due to a high sensitivity to conductive borehole fluids, and a shallow depth of investigation.
U.S. Pat. No. 5,508,616 (Sato, et al.) describes a directional induction tool for wireline logging with inclined coils rotated by a motor that can be used to map conductivity variations around the borehole. Many other earlier patents describe similar schemes using stationary orthogonal coil arrays to provide directional information about conductivity anisotropy (for example, see U.S. Pat. No. 3,808,520 (Runge), U.S. Pat. No. 4,302,723 (Moran), and U.S. Pat. No. 4,360,777 (Segesman). In general, these methods are not adaptable to MWD, because they do not solve the drill-collar conductivity problem.
A method of borehole logging at high frequencies for MWD or wireline employing reflectors with antenna elements to perform directional measurements is described in U.S. Pat. No. 5,530,359 (Habashy, et al.). This patent discloses a subsurface radar application, with a transmitter antenna at a spaced-apart distance along the tool axis and a set of receiver antennas placed around the periphery of the tool. A simultaneous sensing in all radial directions is thus achieved, and by a solution of a time-difference or a phase-difference equation the direction of a reflecting anomaly in the surrounding rock may be found. The method does not measure the conductivity of the anomaly or of the surrounding rock.
Various MWD antenna designs with antenna apertures that modify the reception pattern are described in U.S. Pat. No. 4,940,943 (Bartel, et al.) and U.S. Pat. No. 5,157,331 (Smith.). Means for encapsulating and protecting coil antennas for MWD are given in U.S. Pat. No. 5,661,402 (Chesnutt, et al.) and U.S. Pat. No. 5,212,495 (Winkel, et al.), but all these methods refer to tools of the "wave propagation" type operating at frequencies close to 2 MHz, and none are truly directional.
In U.S. Pat. No. 5,644,231, Wignall describes a method of using magnetic cores in a wireline tool and means to protect and enclose them to minimize the effects of high pressure and borehole fluid invasion. Finally, in U.S. Pat. No. 4,651,101, Barber et al. describe methods for building a non-directional induction wireline tool with a metallic supporting structure that passes through the axis of the coils. (All of the patents discussed in this background section are hereby incorporated herein by reference.)
None of these prior logging tools provide, alone or in combination, an apparatus that is suitable for obtaining directional resistivity information near the bit while a well is being drilled, without being adversely effected by the mass of conductive metal in the drill collars. Such a tool would be desirable to provide real-time directionally focused information regarding nearby geological and fluid boundaries during directional drilling operations.